Spectroscopic analysis of a gas mixture excited by a high-frequency electric field



March 1954 c. F. ROBINSON SPECTROSCQPIC ANALYS IS OF A GAS MIXTURE EXCITED BY A HIGH-FREQUENCY ELECTRIC FIELD Filed June 4, 1949 POWER MR W M N W W m 5 z w W M M m .w a 6 IIII/ lfl/ 1/ Fvwtw w 6 MN v 7 5 a Wm u 6 n n I. n I z m m M m Patented Mar. 2, l 954 A 'SPECTROSCOPIC ANALYSIS OF A GAS- MIX- TURE EXCITED BY A HIGH-FREQUENCY ELECTRIC FIELD Charles F. Robinson, Pasadena, Calif., assignor to Consolidated Engineering Corporation, Pasadena, Calif., a corporation of California Application June 4,: 1949, Serial No. 9 7,255

This invention relates to spectroscopic analysis J 5 Claims. (01. 8814) of gases and vapors and to apparatus for carrying out such analysis.

Ionization of gas may be produced in several ways as for example by electron bombardment. To accomplish ionization, a certain amount of work must be done at the expense of the kinetic energy of the bombarding electron. The amount N of work, or the electron energy required to ionize a given molecule is expressed as the ionization cule. For example the ionization potential of O2 is volts. This means that the anode voltage of an electron gun employed to'develop the bombarding electrons must be at least 15 volts l0 potential or the ionization voltage of that mole-' before the electrons emitted will have sufficient 7 energy to ionize any oxygen molecules with which theycollide. When an electron collides with a gas atom with insuflicient energy to produce ioniz'at'ion,"an electron of-the atom may be displaced to an energy level different from the one it normally "occupies absorbing a portion of the energy of the colliding electron in so doing. When the displaced electron falls back to its normal energy level it gives off the energy it absorbed in displacement in the form of electromagnetic radiation. All atoms have many characteristic dischargefrequencies which form the well'known line spectra of-gases. The energy required to bring about this electron-displacementand subsequent radiation is known as the excitation o-- tential. Since v it takes less energy to displace an electron into an'excited energy levelthanit does to split-off the electronfrom the atom, it

follows'that the excitationpotential of a given" gas is lower than its ionization potential. Where-- asth'e ionization potential of H2 is 16 volts,- its first excitation potential is only'11.5'7'volts.

The conventional emission spectrograph makes use of thecharacteristic discharge spectrum of various materials as 'ameansof analyzing gase ousor solidmixtures. The present invention is directed primarily to'the analysis of gases and emphasis is accordingly placed on such'analysis. In'a conventional emission-spectrograph a D. C. or'low-frequency A. C. field is established between two spaced electrodes. =A gas-sample to be analyzed isintroduced between the electrodes and the" potential difierence'is raised above the first"excitation potential and generally above the ionization potential of all of the gases suspected" of being in the sample. By examining the spectrum of the resultant discharge radiation} the components of the sample can be detected fromthe appearance of the characteristic frequency lines. However, there are limitations on the-scope of-emission spectroscopy'which can best be made apparent by an exam ale, for instanceidetection' OrLQmces'bf helium in other gases. e r. The problem of detecting extremely small traces of helium as a contamination in other gas has achieved major significance in recent years as a result of the ever increasing use of helium in leak detection. A widely practiced method f or detecting leaks in closed vessels or systems is to evacuate the system and to analyze the gases evacuated from the system. By playing a stream of gas foreign to the atmosphere within the sys-' tem over the exterior of the system, the existence and location of leaks can be ascertained by the appearance of such gas in the evacuating system. A recent improvement in leak detection techniques involves the more or less reverse procedure of pressurizing the test vessel with a gas constituted in whole or in part of a constituent foreign to the atmosphere surrounding the vessel.

By exploring the exterior surface of the vessel with a sampling probe connected to an analyzing system, leaks are detected by the appearance oi such constituent in the atmosphere drawn into the probe and analytical instrument connected thereto. w I

Helium is possessed of several characteristics which make it practically the ideal gas for use as a tracer in leak detection in accordance with either of the methods outlined above. tural abundance in air is so low as to be 'insignificant. It is inert and relatively inexpensive. Moreover, helium resembles hydrogen in that it will leak from vessels which are substantially leak proof with respect to substantially all other gases except hydrogen'and helium. This last mentioned property permits detection of leaks which, with respect to other gases, might remain latent until after the vessel had been in service. V Helium,'however, in the comparatively small concentrations involved cannot be detected by means of emission spectroscopy as presently practiced. Accordingly, more complex analytical instruments are presently being used. The rea-' sons why helium, in low concentrations cannot bedetected spectroscopically with conventional instruments "are explained as follows.

Helium has a high ionization potential, i. e. 24.5 volts and a high first excitation potential, i. e. 21.12 volts. I n a -conventional gas discharge' containing :a mixture of gases having different ionization potentials, the emission. spectra of gases ,having relatively highrironization poten-. tials tendto be extinguished byaphenomenon knownas collision of the second kind!- Insuch a system, excitedatoms suchas helium tend to transfer their energy of excitation to atoms of lower ionization potentialssuch as-nitrogen (14.6 volts), oxygen (13.6 volts) or mercury (10.4 volts) in preference to releasing thisenergy-by-radia: tion ofv their characteristic frequencies. It isithis do effect'which makes the spectroscopicdetection of Its naexcept motion due to thermalenergies;

3 traces of helium in the presence of other gases difficult.

Another impediment to the spectroscopic analysis of helium is the existence of a so-called metastable state which the excited helium atom is likely to assume at about 20. volts bombarde ment energy. Without explaining the mechanism of the metastable state, sufiice to say that it accentuates the normal tendency of a ga with a high ionization potential. to; lose energy to gases of lower ionization potential in preference to radiation. Further, and in the presence of other gases, helium is diificult to ionize because electrons tend to ionize first those gases of lower ionization potential. The foregoing factors, 1. e. the. tendency towards energy transfer in preference to radiation, the accentuation of this tendency in helium by the existence of a metastable state, and the difficulty of ionizing helium in the presence of gases of lower ionization potential, renders it virtually impossible to detect small amounts of helium by presently practiced spectroscopic means.

If the pressure of the gas being analyzed spectroscopically could be reduced to the point where the probability of collisions between helium atoms and. other gaseous atoms or molecules was suificiently small, the effects of the foregoing factors could be substantially reduced. However, with the conventional D. C. spectroscope, the pressure cannot be reduced appreciably because the mean free path of electrons cannot be greater than the spacing between the electrodes since, if it were otherwise, it would be impossible to obtain a self-sustaining discharge or ionization of the gases.

I have now found that ultra-high frequency discharges may be maintained under conditions of reduced pressure in which the electron mean free path is much larger than the inter-electrode spacing. With such ultra-high frequency, it is possible to maintain a self-sustaining discharge with only traces of gas present between the electrodes, the discharge being maintained largely by secondary emission alone. Hence it is possible to maintain a sufficiently low pressure so that the probability of collision between helium and other gas molecules is so low as to permit spectrum emission from excited helium. Thisultrahigh frequency discharge may be maintained under pressures in which collision of the second kind, as well as all other collision processes, are highly improbable. This in turn means that helium will not experience interference from. other gases in such a discharge. Accordingly, I

have developed an emission spectroscope employing ultra-high frequency discharges which has a sensitivity competitive with other analytical instruments and can be built for a much smaller cost.

In addition to overcoming the factors which prevent helium detection by-D. G. emission spectroscopy, my invention has other advantages. I have found that helium ions receive essentially noenergy from a high frequency field as they do from a D. C. field, hence, the tendency of helium ions to. drift. towards the electrodes in a D. C. field is. eliminatedv and there, is, underfavorable conditions,no migration of; the helium Additionally, at high frequencies. there is developeda so-called space charge trap (net negative space. charge), since there are more electrons in. theregionbetween the electrodes thanthere are-gas eous ions. Thisspace charge trap further tends.

to prevent migration of helium ions, towards the electrodes.

Accordingly, the present invention contemplates the method of detecting helium in the presence of other gases which involves subjecting the gas mixture at. very low pressures to an ultra-high frequency field, obtaining the emission spectra of the gases subjected to the high frequency field and ascertaining the presence of helium by examination of the spectrum. The invention, also. contemplates apparatus for carrying out the aforementioned method which comprises a discharge cavity wherein the gas mixture may be subjected at low pressures to a high frequency field, a window; in the discharge cavity and an, associated optical system and photoelectric or other means. for detecting the presence. ofi helium in the gases subjected. to. the discharge.

Briefly, the, apparatus of the, invention com.- prises the combination of an RF. oscillator, a. discharge cavity containing a discharge, bottlethrough which the gas sample is drawn, an

optical system and means for indicating. the

presence of one or more gases in, the gas. sample.

If. the invention is to be used for the, detection of helium in leak, detection, the; optical system, may be a fixed focus system and, photoelectric.

means may be coupled withindicating means for.

signalling the presenceof helium. in, thegassarrrm ple. If the apparatus of the invention isto be.

used for a more complex analysis, the QpticaL system may be a variable focus. system; and re:-

cording means may be provided for recording the spectrum obtained. Fixed focus. and; variable focus optical systems. are well. known in the-f spectroscopic art.

The invention may be more clearly understoodv with reference to thefollowing detailed (19501111)?- tion taken in conjunction with the. accompanying drawing in which:,

Fig. 1 is a diagrammatic illustration of afixedfocus type of apparatus, in accordance with the. invention; and

Fig.2 is a perspectiye view. of the optical sys.-- tem showing. themeans of focussing a single fre quency on photoelectricdetection means.

Referring to the drawing, the apparatus. in cludes. an RF oscillator it of the cavity resonator type, a discharge cavity 12, anjoptical-system M, a photocell l6, indicating, means. I]. and a. power supply: 18.

Oscillator NJ is a conventional. RFoscillator.

comprising a cathode, 20, a gridzland1 ananoda or plate 22 mounted.- respectively in the,endsof concentric tubes 23;, 24, 25. Conveniently this. assembly comprises-a triode vacuum tube; of.- the. lighthouse type, of which many formsare. commercially available, mounted in.- the cavity tubes. 23-, 24, 25 according toweliknown means. Alternatively the; vacuum tube, maybe a. lighthouse tetrode of atype; commercially, ayailabla,

The tubes 23-, 2-4, 25 form. anannular grid cavity- 2t' and an. annular plate cavity 2].. Tuning plugs 29-, 30-. provide means; for tuning; the; grid; and; plate cavities. respectively. Heating, wires; 3-24 are; carried. through. the; axial tube, 23, to theoath ode 20-.and; a gridresistor- 34 is-connected between; the grid 2-! andthe. cathode 20- according to; methods well, known in the; construction of; RE- oscillators.

The ieh a e-p0w rsun r Itincludes a source-ofvoltagewhich. may be, unregulated., of: suitablevoltageand current. which may be. ap.---- proximately 800 volts and- 100, milliamperes and. which. is.- connected. through a. lead. 3i. to the plate 22 .of the oscillator. A coupling loop 38 i ing plug 48 provides means for tuning the cavity 46. A coupling loop 42 is mounted in the cavity i on the end of the coaxial cable 40 which projects through the tuning plug 48. A space is provided between the upper end of column 45and the top of the discharge cavity-in which is mounted an axially perforated glass discharge bottle 50. The bottle 50 is connected through a passageway in the top of the discharge cavity to a source of gas to be analyzed, say a sampling probe in a leak detector, and through a coaxial passageway 52 in the column 45 to an evacuating system which may include pumps 54, 55. The central column 45 is preferably spring-loaded within the housing 44 to seal the discharge bottle. The desirability of the glass bottle will vary from one application to another depending on the nature of the gas to be analyzed and on the relative convenience of maintaining vacuum seals around the glass bottle or the cavity itself. Usually the glass bottle will be convenient but it is not necessary to the invention.

The discharge cavity l2, as shown, is provided with a permanent magnet 56 mounted in the central column 45. I have found that magnet 55 serves to increase the electron density in the region in which the discharge bottle is located. However, the magnet is not a necessary feature of the invention, the apparatus operating entirely satisfactorily without it. A lens 60 is mounted in a Wall of housing 44 in alignment with the discharge bottle 50.

The optical system [4 includes a light-tight box 62 comprising two compartments 62A, 62B. A lens 63 and a back-silvered prism 64 are enclosed in compartment 62A and the photocell I6 in compartment 62B. The box is mounted in line with lens 60 in the discharge cavity. A first slit 65 in the outer face of the box opens into compartment 62A and admits radiation passing through lens 60 into this compartment. The radiation passes through lens 63 through prism 64, and is reflected back through prism 64 and lens 63 to a second slit 66 opening from the compartment 62A into the compartment 62B. A mirror 61 deflects the rays passing through the second slit to the photocell IS. The photocell I6 is conveniently a conventional photomultiplier tube.

The cathode IBA of the photocell is grounded and the anode I6B is connected through lead. 68

to the high voltage power supply [8. This power supply in the illustrated embodiment includes" a source of 1000 volt 1 milliampere current preferably 1% regulated and limited to 1.5 milliamperes maximum current. which is supplied through line 58 to the anode I6B of the photocell. An ammeter I! connected in series in line 68, indicates the appearance of helium in the gas sample by signaling the periods of conductivity'of the photocell.

A further refinement in the apparatus includes a speaker 10 connected through a transformer H to line 68. By including a small (say 5%) A. C.

component inthe voltage source supplying I line" 68 and the photocell, the speaker 70 will emit ansaudible signal when the photooell conducts.

Since the arrangement is such that the photo 11 cell will conduct only when helium appears in the discharge bottle, the signal will be emitted from the speaker only at such time.-

Theoptical system H is a fixed focus system.- That is, the prism 64 is oriented within the 'box 62 so that only a selected characteristic frequency of the helium spectrum will be passed through the second 's'lit'66 and be reflected by mirror 61 onto the photocell. V

'The operation of the apparatus is as follows: Power is supplied to the RF oscillator wherein a signal of from about 250 to about 750 megacycles is developed. This oscillating current in the plate cavity of the oscillator induces a like oscillating current in the coupling loop disposed in the cavity. This induced current flowing in the companion coupling loop in the discharge cavity sets the discharge cavity into oscillation. which gives rise to a high frequency field across the gap occupied by the discharge bottle.

A gas to be analyzed is drawn into or through the discharge bottle at a pressure of approximately 10* to 10- mm. Hg. It is usually'necessary to admit a burst of gas to raise the pressure initially to perhaps 2-3 l0 mm., after which the pressure may be reduced to the lower value without extinguishing the discharge. Although the discharge may be maintained at pres sures as high as 1 mm. Hg, the, advantages of this type of excitation are not realized at such high pressures. In order to minimize intermolecular and ion-molecular collisions, it is desirable to maintain the pressure as low as possible. Since the pressure is very low, the chance of an'-ex-' cited helium atom undergoing'collisions with other atoms and subsequent energy transfer is reduced appreciably and helium radiation is 0b- 7 tained. The light from the gas discharge is admitted to the optical system-wherein the prism and lens arrangement is such as to focus a single oneof the characteristics helium frequency lines onthe mirror and photocell. When there is no helium in the gas passing through the discharge bottle, no light will fall on the phototube. When helium is present, the phototube will conduct,

this state being registered by deflection of the ammeter or an audible sound from the speaker.

The frequency range in which the method and apparatus of the invention are effective in detecting traces of helium in other gases has a lower limit determined by the point at which oscillatory electron travel is lost. It has been determined by experiment that the lower limit-for practical purposes is about 200 megacycles. Lower frequencies, say of about megacycles.

may be used but tend to require discharge'chambers of excessive volume. As the frequency is increased, the power requirements likewise increase in order to maintain the necessary kinetic energy for ionization of helium. The relationship can be expressed mathematically as fol-- lows; Y

mux gmug 1+ I cosOI) where:

T=.kinetic energy.

E'=Eo sin (wt+0).

q =charge on particle. m=mass of particle, generally an electron.

w=2 1r (frequency in cycles/sec.)

2 tmtmzi:

To; maintam; "If at: a given minimum vral'lueio1:'-

dischar e, any increase in us must: be. accompanied by: an increase. in Ha. For this reason.

imension; a pressure. of. less. than about.

X mm. Hg: is employed.

By: operating. at; or: below a pressure at which; the MEP-of helium ions exceeds the inside. dimens. stem of; the dischar e. bottlei. the probability cl. inter ionic collisions is small. In the: high ir quency field employed; however, the. helium ion oscillates; within the field: and. is not lost by migration. In the. conventional; 13,. C. discharge the ionic: must be consid rably small r than:

the spacing between the; electrodes. to. achieve a self-sustaining discharge. oration at pressures where inter-ionic collisions. effectively extinguish or. suppress the helium spectrum. as. above described-..

Although the; invention has been d scribed.

with particular reference to analysis ofihel'rumr itis. not so limited, the emphasis onhclium being because; of; its, importance in; leak detection SEC-2h? nology and theinability to detec small, amount of this gas by conventional spectroscopic means; Any gaseous mixture can be: analyzed in accord-- ance with the invention. Furthermore, the particular apparatus, shownv is not essential-., Qthcn forms of oscillators. discharge cavities; etc. may be.- usedprovided only that the requ rements; or; frequency and pressure can be met.

I; claim:

1. In a method for analyzing a gas mixtures which involves producing radiation to. be. spec: troscopically analyzed, the improvement: whichcomprises producing in a discharge chamber in. which the mixture is contained a pressure: less: than that, at. which the mean free; path. of; gas ions produced in themixture equals the greatest, dimension of the; chamber, disposing the gas the chamber at said pressure in an alternatin electrical fieldhaving a frequencysubstantially in excess. of 100 megacycles whereby-- radiation; is produced and spectroscopically analyzing said. radiation lac-determine spectral components of the. mixture.v

2; In. spectroscopic analysis, the improvement which. comprises passing-a streamof gas throu h. a chamber at a pressure below that at whichthe mean free path of helium equals the ma mnum dimension of the chamber; producing in. the gas. in the, chamber at said pressure an alternating: electrical field having a frequency of about 290; megacycles. to about 1000 megacycles thereby: producinga. gas discharge. having a spectrum, spectroscopically analyzing the spectrum and;ex-.- amining the analyzed spectrum for lines characteristic of helium.

3. In a method of analyzing a gas mixture which involves producing radiation to be spectroscopii cally analyzed, the improvement which comprises producing in a discharge chamber in which the mixture is contained a pressure less than that at which the mean free pa h. of. gas

Ilhis requires .opr-

ions nroducerl in-thez miature equals; the. greatest t:

dimension: of the chamber; disposing the as; in the chamberat said pressure in an ultra high trequencyalternating electricalgfield where by: radiation isv produced." and; spectroscopically analyzing said; radiation to. determine spectral components. of the mixt nza.

4. In a method for analyzing a gas mixture; for helium which involves: producing radiation to be: spectroscopically analyzed, the improvement: which comprises. producing in a discharge: chamber-in which the mixture isycontained a. pressure. loss than that at which the. meanfree path. of helium ions equals; the largest.

dimension of the chambercontaining the mix ture, establishing in the chamber an alternatr ingelectrical fielct having a. frequency or from about 200 megacycleatov about, 190.0. megacycles... and of a field strength; snfii'cierltv to.- impart; at" least 25; electron volts to; an.- electron whereby radiation is; produced and; spectroscopically an.

alyzmg; the radiation. to; determine. the. presence of spectral: lines. characteristic ofi' heliurm 5'. In a method for analyzing; a gas mixturev which involves. producing radiation to. be spectroscopically analyzed; the improvement which comprises detecting helium. in. the: mixture by creating in the; mixture at. a pressure, below that. at which thamean free path of a helium ion.- is: equal to. the greatest; dimension of the chamber an. alternating electricab field having a frequency of at leastZOO mcgacycles; and thereby producing an electrical discharge accompaniedby'radiation, and spectroscopically: analyzing that radiation to. determine. the presencev of lines characteristic of the: helimnspectrum.

F; ROBINSON.

fiefierenccsx Gited in. he fil f: this; patent, UNITED STATES PATENTS OTHER REFERENCES Spectroscopy... Text by Baly; vol. 11, page: 200.

Published by Irmgmans, Green 8: C0,, .55; Fifth. ve" New-York,.192;7..

Ri erche Snet roscopichewol. lihebruary 19%,-

Na. pa es. 20c through 2.09.- and 3; plates. be ween pages 2,04- and 2.05.. Laboratories; Astrofisico.

Experimental- Spectroscopyby Sawyer, pages.

25, 26; and 2.7... Published by- Prentice-Hall Inc, l New York .94.6.

.Bractical; Spectroscopy, H rrison et a1.,. pa es:

190, I91 and 192; 1948. Published by Prentice Hall Inc,., New York, New York.

Printed in. Vatican Ci ll. 

